This project has as its basic goal the measurement of changes in thermodynamic quantities accompanying reactions in biochemical systems. The principal methods used are calorimetry and absorption and fluorescence spectrophotometry. The combination of calorimetric and equilibrium measurements as a function of temperature and medium adds needed information about protein-ligand interactions. Measurements are made upon native and chemically modified proteins. Expanded knowledge about energy changes accompanying biochemical reactions is valuable in understanding control mechanisms in cells. BIBLIOGRAPHIC REFERENCES: M.T. Barnhill Jr. and C.G. Trowbridge, "Reaction Heat Variation with pH in Formation of the Trypsin-Soybean Inhibitor Complex" J. Biol. Chem. 250, 5501-5507 (1975). B.Y. Yung and C.G. Trowbridge, "Resolution of alpha and beta Anhydrotrypsin by Affinity Chromatography", Biochem. and Biophys. Res. Comm. 65, 927-930 (1975).